The reports attribute the alleged casualties to a recent US airstrike and put the figures between 100 and over 600.

“We urge not to use media reports as a primary source in that case. Let’s understand that there are a lot of our compatriots in many countries worldwide, and it’s quite difficult to have detailed information. All the media reports, though, need to be verified.”

“We recommend in that case to reach out to the Defense Ministry, since here we’re dealing with information published in media. We can only operate data on the servicemen of Russian Armed Forces, who take part in the operation of the Russian Airspace forces supporting the Syrian Army. We don’t have information on any other Russians who might be in Syria,” Kremlin spokesmanDmitry Peskov said.

US Defense Secretary James Mattis also said he has no information that Russian army servicemen were among the casualties following the US airstrike.

“I don’t have any reporting — that some Russians — non-Russian federation soldiers, but Russian contractors were among the casualties. I can’t give you anything on that. We have not received that word at Central Command or at the Pentagon,” he told a media briefing on Tuesday.

Mattis called the situation “perplexing,” noting that the reported attack on the SDF base, which allegedly brought about the deadly American retaliation, “makes no sense, it does not appear to be anything coordinated by the Russians.”

Earlier on Monday, one of the Russian presidential candidates, Grigory Yavlinsky, urged Russian President Vladimir Putin to provide a “report” on “numerous” Russian nationals that were allegedly killed in Syria. The veteran Russian politician, who has been unsuccessfully running for president several times since the 1990s, did not provide any source for the claims, apart from media and social media reports.

“Over the past few days, all the world media and multiple social media sources are reporting on numerous Russian citizens killed while fighting in Syria,” Yavlinsky said in a statement, urging Russian leadership to explain the alleged casualties, and even to end the Syrian campaign altogether.

The Kremlin spokesman said he doubts Yavlinsky’s statement is based on “any more reliable sources” than unconfirmed media reports.

A number of reports on social media began circulating this week, suggesting that anywhere from “several dozens” up to 644 Russian “mercenaries” were killed in clashes with US-backed Kurdish forces in the Syrian province of Deir ez-Zor last week. The reports of alleged Russian casualties were picked up by various US outlets, including Bloomberg, The Daily Mail, The New York Times and others.

A February 10 post on Russian social media network Vkontakte (In Contact) said that 196 Russians, including military contractors and special operations forces, were killed outside Deir ez-Zor in US-led coalition strikes using Lockheed AC-130 Spectre planes, helicopters and heavy artillery. The attack was reportedly called for by the Syrian Democratic Forces (SDF) militia and lasted for about four hours.

Audio recordings emerged on WhatsApp, purporting to be calls between unidentified individuals describing an attack by Russian contractors that were thwarted by the SDF who got help from US firepower after they “raised the American flag.” The death toll cited in the recordings ranges from 177 people killed in one squadron to 200-215 people.

One of the first to report the alleged casualties was the former commander of the rebel militia in breakaway Donetsk republic in Ukraine and head of the Novorossiya movement Igor Strelkov. Strelkov wrote on his Vkontakte page on February 10 about “several hundred” killed and injured Russian mercenaries. Citing unverified preliminary reports “from Syria” he claimed that as many as 644 could have been killed. However, he casts doubts on the reports, noting that the whole column, which was alleged bombed out in the US attack, counted “a bit more than 500 people.”

Leader of the Cossack forces in Russia’s Kaliningrad exclave Maksim Buga put the number of people killed at “dozens,” while speaking to Reuters.

There have been doubts about the accuracy of the death toll, with many commentators deeming it inflated and pointing to discrepancies between different accounts.

Earlier, the US military said that it killed more than 100 Syrian militiamen allied with the government of President Bashar Assad after some 500 fighters targeted “well-established Syrian Democratic Forces headquarters” in Deir al-Zor province in what the Pentagon called an “unprovoked attack.”

In a statement last Wednesday, the US coalition spokesman Colonel Thomas F. Veale refused to go into details about the possible make-up of the militia, saying they would “not speculate on the exact make-up of the Syrian pro-regime forces who conducted the attack.”

The Syrian government denounced the attack as “new aggression” and “an attempt to support terrorism.”

The Russian Defense Ministry later said that the Syrian militia came under attack while conducting a reconnaissance operation that was not coordinated with the Russian side, estimating there were 25 casualties resulting from the US-coalition airstrikes.